1. Field of the Invention
The invention relates to fluid application devices and methods, and in particular, to devices and methods for transferring droplets of fluid, such as ink, to a substrate.
2. Description of Related Art
Various fluid application technologies, such as printing technologies, are being developed. One such technology uses focused acoustic energy to eject droplets of marking material from a printhead onto a recording medium. This application is called acoustic ink printing (AIP) and is described in a number of U.S. patents, including U.S. Pat. Nos. 4,308,547, 4,697,195, 5,028,937 and 5,087,931, the disclosures of which are incorporated herein by reference.
Acoustic ink printheads typically include a plurality of droplet ejectors, each of which launches a converging acoustic beam into a pool of liquid ink. The angular convergence of this beam is selected so that the beam comes to focus at or near the free surface of the ink, that is, at the liquid/air interface. Printing is performed by modulating the radiation pressure that the beam of each ejector exerts against the free surface of the ink, to selectively eject droplets of ink from the free surface.
More particularly, modulating the radiation pressure of each beam causes the radiation pressure to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of the surface tension at the free surface. Individual droplets of ink are ejected from the free surface of the pool of ink on command, with sufficient velocity to deposit them on a nearby recording medium.
Various printheads for acoustic printing also are being developed. Page-width linear and two-dimensional lens arrays for line printing are known, as are linear and two-dimensional arrays for multi-line raster printing. U.S. Pat. No. 4,751,530, the disclosure of which is incorporated herein by reference, discloses examples of such printheads.
With fluid deposition technologies such as AIP, selectively depositing marking fluid across an entire recording medium requires moving the printhead and the recording medium relative to each other until the printhead has traversed the entire recording medium. With the printheads illustrated in FIGS. 4A-4B of U.S. Pat. No. 4,751,530, for example, printing is not completed until the entire recording medium has moved past the printhead. Alternatively, with the printheads of FIGS. 4C-4D, printing is not completed until the printhead has scanned back and forth across every swath of the advancing medium. The speed with which such printing devices can print, therefore, is limited considerably.
As a further disadvantage, the physical spacing between the ejectors of many printheads limits the minimum distance between the spots printed by the printheads. In other words, because the minimum physical spacing between ejectors is limited, the minimum spacing between spots also is limited. Crosstalk between ejectors, heat transfer problems and other obstacles limit the minimum interejector spacing achievable on a printhead. Therefore, the number of spots per inch printable on the medium and thus the resolution of the printed image is limited by the number of ejectors that can be packed onto the printhead.